Method and tool for the machining of workpieces with cooling

ABSTRACT

The invention relates to a method for the cutting or shaping working of metallic or ceramic workpieces and the use of a preferably submerged tool, whereby during the machining process a cooling agent, comprised at least mostly of carbon dioxide (CO 2 ), is supplied to the working position. The invention further relates to a tool for carrying out said method. According to the invention, the cooling in particular of submerges tools may be improved, whereby liquid CO 2  under pressure is supplied internally through the tool and released from the tool in the direct vicinity of the actual machining position, expanding through a pressure drop to atmospheric pressure, to give a cooling flow, comprising cold gas and ice particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the cutting or shaping machiningof metallic or ceramic workpieces using a preferably submerged tool, inwhich, during the machining operation, a coolant which at leastpredominantly comprises carbon dioxide (CO₂) is fed to the machininglocation.

The invention also relates to a tool for carrying out the methoddescribed above.

2. Background Description

The method described in the introduction can be found in DE 43 26 517C2. This document discloses a method for the cutting machining ofmetallic workpieces and also ceramic surfaces, in which, as a result ofa coolant being fed to the machining location, cooling is carried outduring the machining operation, and in which, for cooling purposes, acoolant jet which predominantly comprises carbon dioxide and includescold gas and snow particles, is fed to the machining location. Thecoolant jet is obtained from gaseous CO₂ which is under a suitablesuperatmospheric pressure, in such a manner that the CO₂ gas, via a slotnozzle or other slot-like opening, is first of all expanded into anexpansion volume which is formed around this expansion slot and issubstantially closed off from the environment, and the cooling jet isformed starting from this expansion volume and its outlet opening and isdirected onto the region which is to be cooled. In this case, thestarting pressure level for the CO₂ prior to the expansion is at least50 bar.

DE-B 15 509 Ib/49a has disclosed a method for the cutting machining ofmetallic workpieces, in which, as a result of a coolant being fed to themachining location cooling is carried out during the machiningoperation. The coolant used is a liquid jet of CO₂ directed onto theworkpiece via a nozzle. The liquid jet of CO₂ strikes the workpiece atthe machining location, forming a deposit of solid CO₂. The liquid CO₂is intended to expand at the location where the tool touches theworkpiece.

DE-AS 1 037 808 has disclosed chip-forming machining using carbonic acidin the form of snow for cooling purposes. In this case, pressurizedliquid carbonic acid flows out of a capillary tube or a nozzle and as itemerges, as a result of the pressure drop, is instantaneously convertedinto a mixture of vapor and snow.

The cooling method based on carbon dioxide is a dry cooling method.Since at normal ambient temperature carbon dioxide adopts a gaseousstate, no coolant residues remain behind on the workpiece following thecooled machining.

DE 199 15 619 A1 has described a method for discharging machiningproducts of a machining process, in which solid CO₂ particles are fed toa gas stream. The gas stream together with the solid CO₂ particles whichit has collected is then fed to a machining space, where it is directedonto a region in which a workpiece is being machined by a tool, takingin at least the majority of the machining products which are generated.

The article “Kühlschmieren beim Zerspanen” [Cooling and lubricationduring machining] by Kurt Häuser (Technische Rundschau No. 25, Jun. 19,1970, pages 21 and 23; Technische Rundschau No. 26, Jun. 26, 1970, pages29 and 31) teaches the person skilled in the art to use CO₂ as coolinglubricant which is to be sprayed onto the working location as a liquidjet at a high pressure (50-70 atmosphere above atmospheric pressure),where the expansion to form a gaseous coolant takes place, so that thecooling lubricant precipitates as snow at the working location.

U.S. Pat. No. 3,971,114 refers in general terms to cryogenic coolants,in particular Freon-12 as an example of an expandable gas coolant. Inthis case, the coolant, which is passed through a tool, is to emergefrom the tool at a small opening or bore, the intention being for thisbore to control the quantity of coolant; the bore is located at adistance from the actual machining location which is such that chipscannot have any adverse effect on the emerging stream of coolant.

SUMMARY OF THE INVENTION

The invention is based on the object of improving the method describedin the introduction in terms of its tool cooling and of developing atool which is suitable for carrying out the method which has beenimproved in this way.

Working on the basis of the method described in the introduction, thisobject is achieved, according to the invention, by virtue of the factthat pressurized liquid CO₂ is passed internally through the tool and,in the immediate vicinity of the actual machining location, is expandedout of the tool into the machining location as a result of a pressuredrop to ambient pressure, so as to form a coolant flow which containscold gas and snow, the expansion being effected through an expansionnozzle which forms the coolant outlet from the tool.

With regard to the tool, the object of the invention is achieved by atleast one internal coolant channel which is designed for the liquid CO₂to pass through and opens out into an expansion nozzle for theevaporation of CO₂ in the vicinity of a tool cutting edge or atool-guide strip.

The tool may, for example, be a drill, a thread-forming tool, a millingcutter, a boring bar, a reaming tool, disposable tips for lathes or thelike. These may be rotating, stationary and in particular submergedtools with a geometrically defined and/or undefined cutting edge. Thetools may be chip-forming tools or material-shaping tools, for examplemay be in the form of thread-forming tools, where material is deformedby means of impact extrusion. The cutting tools may be tools with one,two or more cutting edges. In this context, the term “tool” is also tobe understood as meaning, for example, an annular nozzle which is usedfor dry broaching (internal broaching) and in which the broaching toolis wetted from the outside or acted on by CO₂ during the process.

In the case of rotating, submerged or other tools, according to theinvention a spindle and/or a tool holder with an internal passage forthe liquid CO₂ and loss-free coupling to the tool is used. The liquidCO₂ is then passed via a connecting line for example through a rotatingspindle of a machining center to the tool without losses. Only when itemerges in the vicinity of the cutting edge via a nozzle with a verynarrow cross section does the expansion take place, in the machiningregion. The CO₂ used for cooling is provided, for example, usingcommercially available gas cylinders and is therefore at a temperatureof approximately 20° C. at a pressure of approximately 57 bar. Thespindle or tool holder is therefore not affected by the expansionrefrigeration.

Suitable tool holders include shrink-fit chucks, hydraulic expansionchucks or collet chucks for all drills, boring tool or threading toolshanks or for tool bodies, e.g. monobloc tools (reamers, boring bars)and tool holders for turning operations and broaching tools.

The method according to the invention is particularly suitable forsubmerged tools, in which the methods described in the introductionsimply cannot be used or can only be used with reduced coolingefficiency. The fact that according to the invention coolant is supplieddirectly at the working location in this case results in a significantdrop in the cutting or shaping temperature. The thermal influences canbe controlled, which has positive effects on the tool service life, thecutting parameters and the workpiece quality, in particular in terms ofdimensionsal accuracy and surface quality.

It is expedient if the coolant flow is only actuated by means of aCO₂-resistant solenoid valve during the working time. In this case, itis advantageous if the expansion nozzle and/or its associated coolantchannel is/are dimensioned in such a way that only the quantity ofliquid CO₂ which is required for sufficient cooling per unit time isexpanded.

Use of the invention makes it possible to eliminate the problems whichhave hitherto occurred with what is known as dry machining(contamination of the machine; thermal problems, in particular wheretight gauge tolerances are important). It has been possible todemonstrate by tests that when the method according to the invention isused, it is possible to maintain a constant temperature at the tool andworkpiece. Consequently, when the cooling of the invention is employed,it is possible for production lines to run completely dry, even where itis necessary to achieve high levels of manufacturing accuracy.

Once it has been possible to establish by extensive and intensive teststhat, at particularly high machining speeds and advance rates, inparticular when machining Si-containing aluminum alloys, it is notpossible to dispense with residual lubrication, according to theinvention in such cases it is proposed to add a low-viscosity lubricant,e.g. a low-viscosity oil, to the liquid CO₂, in which case theinoculation with the lubricant can be performed in the form of acontinuous feed just before the liquid CO₂ stream is introduced into thetool. In this case, intensive mixing is expedient, which according tothe invention can be guaranteed by passing the CO₂/lubricant mixturethrough a small sintered plate.

The liquid CO₂ stream fed to the tool may be at a pressure ofapproximately 55-63 bar and a temperature of approximately 18-25° C. Themixing ratio of liquid CO₂ to liquid lubricant may, for example, be setto approximately 30:1.

In terms of the configuration of the tool, it is advantageous if theoutlet opening of the expansion nozzle lies at a short distance ahead ofthe tool outlet, allowing the tool cutting edge or tool-guide strip tobe reground or sharpened.

As an alternative, it is also possible for the outlet opening of theexpansion nozzle to lie behind the tool cutting edge or in the chipspace.

Furthermore, it may be expedient if the CO₂ expansion nozzle isintegrated in a compressed-air nozzle, which has a lubricant feedopening out into its compressed-air jacket which forms during operation.In this case, the lubricant is then admixed with the compressed-airjacket which forms around the CO₂ expansion jet, in which case thelubricant quantity could be controlled independently of the demand forcooling from the CO₂. However, a solution of this nature is onlysuitable for relatively large tools.

If permitted by the type of tool production it is expedient for theexpansion nozzle to be worked, for example sintered, into the toolduring production of the tool. If the tool is itself produced bymachining, as is the case with boring bars, for example, the expansionnozzle may be installed from the outside or may itself be produced bychip-forming machining or may be introduced into the tool by erosion.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing diagrammatically depicts an embodiment of the inventionwhich serves as an example. In the drawing:

FIG. 1 shows a side view of a twist drill, partly in longitudinalsection,

FIG. 2 shows a plan view of the twist drill illustrated in FIG. 1, and

FIG. 3 shows a flow diagram for the CO₂ lubricant inoculation.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The figures illustrate a tool 1 in the form of a twist drill with twotool cutting edges 2. Two coolant channels 3 which are designed forliquid CO₂ to pass through lead through the tool 1 and, in the vicinityof the associated tool cutting edge 2, in each case open out into anexpansion nozzle 4 to evaporate the CO₂. The outlet opening 4 a of eachexpansion nozzle 4 lies at a short distance ahead of the tool outlet forthe evaporated CO₂, allowing the tool cutting edge 2 to be reground orsharpened.

FIG. 3 shows an exemplary embodiment of the way in which a liquidlubricant is fed into the CO₂ stream before entering the tool.

From a CO₂ tank 5, the liquid CO₂, which may, for example, be at atemperature of 20° C. and a pressure of 57 bar, is introduced into atool 1 via a CO₂ line 6, to which a gas line 7 is also connected, and arotary passage 8. An installation 9 for inoculating this CO₂ stream witha liquid lubricant is diagrammatically depicted by dot-dashed lines.This installation comprises a hydraulic pump 10 which is equipped with abypass and feeds lubricant out of a lubricant tank 11 via a needle valve12 into the CO₂ line 6 at a pressure which at least corresponds to theCO₂ pressure in the CO₂ line 6. Downstream of this needle valve 12, asseen in the direction of flow, a small sintered plate 13 is connectedinto the CO₂ line 6; the CO₂/lubricant mixture flows through this smallsintered plate 13, producing intensive mixing of the two mixturecomponents.

1. A method for machining workpieces using a submerged tool, in which,during a machining operation, a coolant which at least predominantlycomprises: directing carbon dioxide (CO₂) to the machining location,passing pressurized liquid CO₂ internally through the tool, andexpanding the pressurized liquid CO₂ in the immediate vicinity of themachining location out of the tool into the machining location as aresult of a pressure drop to ambient pressure, so as to form a coolantflow which contains cold gas and snow, the expanding being effected inthe tool through an expansion nozzle which forms the coolant outlet fromthe tool.
 2. The method as claimed in claim 1, further comprising usingat least one of a spindle and a tool holder with an internal passage forthe liquid CO₂ and loss-free coupling to the tool.
 3. The method asclaimed in claim 1, further comprising actuating the coolant flow onlyby means of a CO₂-resistant solenoid valve during a working time.
 4. Themethod as claimed in claim 1, wherein at least one of the expansionnozzle and its associated coolant channel are dimensioned in such a waythat only the quantity of liquid CO₂ which is required for sufficientcooling per unit time is expanded.
 5. The method as claimed in claim 1,further comprising adding a low-viscosity lubricant to the liquid CO₂.6. The method as claimed in claim 5, wherein the lubricant used is alow-viscosity oil.
 7. The method as claimed in claim 5, wherein thelubricant is added to the liquid CO₂ stream continuously before thelatter is introduced into the tool.
 8. The method as claimed in claim 5,further comprising feeding the lubricant into the liquid CO₂ via aneedle valve at a pressure which at least corresponds to the CO₂pressure.
 9. The method as claimed in claim 5, wherein after thelubricant has been fed into the liquid CO₂, the CO₂/lubricant mixture issubjected to intensive mixing.
 10. The method as claimed in claim 9,wherein the CO₂/lubricant mixture is passed through a small sinteredplate in order for it to be intensively mixed.
 11. The method as claimedin claim 1, wherein a liquid CO₂ stream fed to the tool is at a pressureof approximately 55-63 bar and a temperature of approximately 18° C.-25°C.
 12. The method as claimed in claim 5, wherein a mixing ratio ofliquid CO₂ to liquid lubricant is set to approximately 30:1.
 13. Themethod as claimed in claim 1, further comprising adding lubricatinggranules to the liquid CO₂.
 14. A tool for carrying out the method asclaimed in claim 1 comprising at least one internal coolant channelwhich is designed for the liquid CO₂ to pass through and, in thevicinity of a tool cutting edge or a tool-guide strip, opens out into anexpansion nozzle to evaporate the CO₂.
 15. The tool as claimed in claim14, wherein an outlet opening of the expansion nozzle lies at a shortdistance ahead of the tool outlet, allowing the tool cutting edge ortool-guide strip to be reground or sharpened.
 16. The tool as claimed inclaim 14, wherein an outlet opening of the expansion nozzle lies behindthe tool cutting edge or in the chip space.
 17. The tool as claimed inclaim 14, wherein the expansion nozzle is worked, into the tool duringproduction of the tool.
 18. The tool as claimed in claim 14, wherein theexpansion nozzle is integrated in a compressed-air nozzle, which has alubricant feed opening out into its compressed-air jacket which formsduring operation.
 19. The tool as claimed in claim 14, wherein theexpansion nozzle is sintered into the tool during production of thetool.
 20. The method as claimed in claim 1, wherein the expansion beginsin the tool through the expansion nozzle incorporated into the tool.